THE BLAST FURNACE. .4:5 • siderable gain in fuel,* but it is a matter of great doubt whether there is any important saving in the long run. The Middlesbor- ough furnaces should profit more than others, as they carry twice as much stone as most American furnaces, but the practice has -made little headway in that district. One reason for the-failure is that the ordinary methods of burning lime do not expel all the gas, so that only a- part of the benefit can be expected. Another -reason, lies in the fact that when burned lime is put into the blast 'furnaces it is exposed to the action of carbonic acid gas (C02), and, although, this gas is expelled from stone at a red heat, it is absorbed again at a lower temperature, so that immediately after being charged into the furnace, this burned lime reverts to the condition of limestone, which sinks down with the charge and acts in the •same manner as if it had never been burned. SEC. TTg.—The blast.—On another page, under the discussion of •Tunnel Head Gases, are given calculations on the amount of air needed for a furnace and on the heat required to bring it to-thjB desired temperature. In America, a temperature of 1000° to-1100° F. is often considered sufficient, and on Mcsabi ores a- higher heat is believed to give trouble from slips. la foreign countries higher temperatures are maintained. It is a common practice abroad to have several furnaces on one common air main, but the modern method is to have an independent engine for each furiiace •in order that a constant quantity of air be forced into the tuyeres 'without any regard to the resistan.ee caused by internal conditions. Let it be-arbitrarily assumed that a coke fire with cold blast will give a temperature of 2500° :F., and that if the blast be heated to 1000° F. a temperature of 2900° F. will be obtained. If, then, it is necessary to melt 100 pounds of a metal that fuses at 2700° F., it might be possible to do so with 100 pounds of coke with hot blast, when it would be impossible to do it at all with cold blast. In this case the heating of the air to 1000° F. has worked a revolution in fuel economy, but it by no means follows that an increase to -1100° or 1200"9 will save much more, for if 1000° is sufficient for the work in hand, an increase beyond that point may be of little value. These arbitrary assumptions illustrate the use of hot blast in furnaces, for it was the first step that produced the revolution by obtaining a temperature that changed all the operating conditions. *Journal I. & S. I., Vol. I,' 1898, p. 60.